1. Field of the Invention
The present invention relates to a mobile communication system capable of providing a multimedia broadcast/multicast service (MBMS). More particularly, the present invention relates to a method for resuming header decompression in packet data with compressed headers.
2. Description of the Related Art
UMTS (Universal Mobile Telecommunication Service) is typically based on GSM (Global System for Mobile communication) and GPRS (General Packet Radio Services) standards. In contrast to the GMS which uses TDMA (Time Division Multiple Access), the UMTS employs a wideband CDMA technology and provides packet-based text, digital voice or video signals, and multimedia data at a high speed, above 2 Mbps regardless of location. The UMTS employs a switched virtual connection, called a “packet switching system” using a packet protocol, such as an Internet Protocol (IP). It is possible to access the UMTS from any end location in a network.
In general, since radio transmission resources are limited, it is inefficient to transmit data at high speeds to various user equipment. For instance, to provide one user equipment with a video streaming service of 64 kbps, radio transmission resources capable of transmitting data of 64 kbps is needed after forming a dedicated channel for the user equipment. If there are n user equipments, the amount of radio transmission resources needed increases by n times.
Alternatively, service can be provided to plural user equipments through a common channel. In this case, the same service can be provided to the user equipments through the common channel by using the same radio resources regardless of n if the user equipments are located in the same cell, so radio resources can be used effectively. The MBMS has been developed based on this point of view. That is, the MBMS has been developed in order to effectively transmit the same data to a plurality of user equipments in the UMTS. The MBMS is efficient and effective when transmitting multimedia data which requires a great amount of radio transmission resources.
To provide a packet-based multimedia service, a real time transport protocol (RTP), a user datagram protocol (UDP) and an Internet protocol (IP) are typically used. When using a combination of the above protocols, a header size of one packet may exceed 60 bytes, so it is necessary to compress the header to reduce consumption of the radio transmission resources. In the MBMS, a robust header compression (ROHC) scheme is used for compressing the header. Compression and decompression of the header is managed by a packet data control protocol (PDCP) entity, which manages the protocol of a PDCP layer. In a particular system, the PDCP entity is provided in a radio network controller (RNC), which manages a plurality of cells.
As mentioned above, in the case of the MBMS, user equipment requesting the same MBMS can be located in a plurality of cells, requiring MBMS data to be transmitted to all the cells. The system may have a PDCP entity for each cell or a common PDCP entity for several cells. If the PDCP entity is changed because the user equipment moves between cells with mutually different PDCP entities, the user equipment reestablishes its connection with the new PDCP entity.
Reestablishing the PDCP entity connection usually requires a new header decompressor and header decompression context. To do so, header information required for header decompression is obtained. However, it is very inefficient if the user equipment waits for the header information. Thus, it is desirable to provide a technique allowing the user equipment to rapidly resume header decompression without performing an initialization. When the PDCP entity connection needs to be reestablished from movement between cells.